Patches on Sand

Hello - can someone advise on these greenish patches on the sand please? The patches come up every morning. Bear in mind the sand was “hoovered” less than 24 hours ago.

Is it something that will spread / needs to be treated?

Tested water today and all is fine.

Thanks

Looks like Diatoms.

Is it a fairly new tank or new sand? Diatoms are super common in new setups, especially ones with sand. They are a type of algae but are a bit unique in that they use silicates to grow. That's why they are common in new tanks as there are a lot of silicates available.

They will eventually burn themselves out and go away.

Blue green algae (Cyanobacter bacteria).

Reduce the amount of dry food going in the tank.
Increase water changes and aeration.
If you have old light globes above the tank, replace them with new globes that have a 6500K rating (K is for Kelvin).
Add some floating plants.